Fix a deadlock issue that would happen when loading an AppleTV or watchOS binary.

This was a regression that was caused by svn revision 269877:

commit 1ded4a2a25d60dd2c81bd432bcf63b6ded58e5d6
Author: Saleem Abdulrasool <compnerd@compnerd.org>
Date:   Wed May 18 01:59:10 2016 +0000

    remove use of Mutex in favour of std::{,recursive_}mutex
    
    This is a pretty straightforward first pass over removing a number of uses of
    Mutex in favor of std::mutex or std::recursive_mutex. The problem is that there
    are interfaces which take Mutex::Locker & to lock internal locks. This patch
    cleans up most of the easy cases. The only non-trivial change is in
    CommandObjectTarget.cpp where a Mutex::Locker was split into two.
    
    git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@269877 91177308-0d34-0410-b5e6-96231b3b80d8

This change actually changed the Platform::m_mutex to be non-recursive which caused the regression.

<rdar://problem/29094384>

llvm-svn: 286908